1. Field of the Invention
The present invention relates to a light emitting device and a method of fabricating a light emitting device, and more particularly, to a semiconductor light emitting device having a short luminous wavelength, typically a nitride semiconductor laser device and a fabricating method thereof.
2. Description of Related Art
Because of their band structure and chemical stability, nitride semiconductors (e.g., AlN, GaN, InN, or the like, or solid solutions of them such as AlGaN, InGaN, or the like are included. They are collectively called nitride semiconductors in this application.) that are compounds of Al, Ga, In, or the like, which are group III elements, and N, which is a group V element are expected to be applied as materials for light emitting devices or power devices, and applications of these nitride semiconductors to light sources of information recording apparatuses are also attracting attention
A laser chip which is made of such a nitride semiconductor and used for a laser device is obtained by cleaving and dividing a wafer that is formed by laminating nitride semiconductor layers and an electrode on a substrate. And, formed on an end surface obtained by cleaving a wafer, especially on the light-emitting side end surface, is a protection film (low-reflectance film) which has a single layer of a material (e.g., SiO2 or Al2O3 or the like) transparent to light or a small number of layers obtained by combining such a single layer. On the other hand, formed on the end surface opposite to the light-emitting side end surface is a large-reflectance protection film (high-reflectance film) which includes a multi-layer lamination of Al2O3 and Ta2O5, and the like. With such protection films formed, the reflectance is adjusted to allow an efficient light emission, and the change of properties of the end surface caused by chemical reactions such as oxidation or the like is prevented.
However, there is a problem that such a protection film collects and accumulates pollutants thereon, and the problem is especially notable in light emitting devices that emit light having a short wavelength of 500 nm or less. This problem is caused because siloxane having a combination of Si and O, and hydrocarbon compounds or the like present near the chip of a light emitting device are polymerized by the short-wavelength light emitted from the chip, and adhere to and accumulate on the protection film, which has also been the problem with the light emitting devices which use the above-mentioned nitride semiconductors and emit light having the short wavelength.
This problem that the chip collects and accumulates pollutants thereon is explained based on a schematic side view of a laser chip shown in FIG. 9. A laser chip 100 shown in FIG. 9 has the above-described low-reflectance film 101 and high-reflectance films 102 as the protection films, and the light emitted from an active layer 103 at the end surface on the light emitting side of the laser chip 100 travels through the low-reflectance film 101 and is directed in the direction substantially perpendicular to the end surface as shown by broken lines. At this time, a pollutant 104 reacts with the light emitted, collects and accumulates on the low-reflectance film to absorb the light emitted. Accordingly, it becomes necessary to increase the driving current to maintain the emission amount of light. Such increase of the driving current shortens the device life and brings the instability of operation of the light emitting device.
As a specific example of this, a result of the performance test of a laser device having the laser chip 100 shown in FIG. 9 is illustrated in FIG. 10. FIG. 10 is a graph when the driving current was so controlled as to maintain the optical output at 15 mW and the temperature at 75° C. in a continuous oscillation of the laser device having an oscillation wavelength of 405 nm. As shown in FIG. 10, because the pollutant 104 in the atmosphere gradually collects and accumulates thick after the oscillation begins, the driving current increases as the driving time passes. Specifically, the current value is about 60 mA immediately after the beginning of the operation, but increases to about 150 mA, that is, more than twofold in 500 hours of the driving time. Besides, the driving current fluctuates up and down in units of dozens of milliamperes, and the drive operation is unstable.
To prevent this problem, taking a can package as an example, a method of curbing the invasion of pollutants by sealing the chip air-tightly with a cap and controlling the ambience sealed air-tightly has been proposed (see patent document 1). Besides, a method of removing pollutants by plasma cleaning before the air-tight sealing, and a method of removing pollutants with the aid of adsorbent placed in the package sealed air-tightly to adsorb pollutants have also been proposed (see patent documents 2 and 3).    [patent document 1] JP-A-2004-289010    [patent document 2] JP-A-2004-040051    [patent document 3] JP-A-2004-014820
However, these methods require not only a strict control over the ambience sealed air-tightly but also a structure for the air-tight sealing, which makes the light emitting device bulky. Therefore, it is difficult to use the device in light sources for information recording apparatuses, typically, optical pickups for CDs (compact disk) and DVDs (digital versatile disk) because of the bulky size of the light emitting device. Although a light emitting device having a frame package that does not need the air-tight sealing can be easily applied to an optical pickup, the pollutant 104 collects and accumulates as the service time passes as shown in FIG. 9, which shortens the device life and brings an unstable operation of the device.
Moreover, even if a type of package in which the chip is sealed air-tightly, because organic adhesives such as Ag paste, silicon based-, or epoxy based-adhesives need to be used in the package, the contents of these adhesives volatilize and become pollutants. It is possible to prevent pollutants from collecting and accumulating to some extent by removing them with radiation of plasma or the like before the air-tight sealing, or by performing the air-tight sealing in a dried air having a dew point of −15° C. or lower, but because the plasma radiation and a strict control over the sealed ambience are required, the fabrication processes become cumbersome. In addition, because pollutants are volatilized into the sealed ambience by the heat generated during the device operation, it is difficult to prevent perfectly the pollutants from collecting and accumulating. Besides, because a process for confirming whether or not the air-tight sealing is surely carried out is necessary, and because the light emitting devices that are not surely sealed air-tightly must be disposed of as wastes, the yield is lowered.